[NEW] Add offline mode

[brad-u410]

This enables operators to still run the API and Node containers, making it easier to keep the node operator key entirely offline.

I'm currently testing this for internal use, but wanted to see if this was worth getting merged upstream.

[jclapis]

This is actually going to be useful for an upcoming refactor we have planned. Offline is kind-of a misnomer if what it really means is "simulate all of the transactions but don't actually sign and submit anything to the network"; if I were to merge this upstream I'd probably replace the name with something like "simulation mode".

[brad-u410]

@jclapis - in my case, the node operator key is kept offline. When it needs to sign transactions, those are transferred to the airgapped machine, signed, exported and broadcast.

However, that caused two issues:

1. Most the of CLI no longer works, even for informational commands
2. I don't know what transactions I would have been automatically signing via the smartnode container, as I can't run that service